FIG. 1 is a block diagram showing a class AB amplifier according to the related art. In FIG. 1, in a situation in which a DC voltage of DC power supply 1070 is applied at power supply terminal 1003a of power amplifier 1003, when an input signal of the amplifier is input to signal input terminal 1003b of power amplifier 1003, a class AB amplifier amplifies the input signal of the amplifier.
FIG. 2 is a circuit diagram showing an example of an internal circuit of power amplifier 1003. In FIG. 2, power amplifier 1003 consists of field effect transistor (FET) 1080 and inductor element 1052 in a situation in which a gate bias circuit and so forth are omitted from power amplifier 1003. Alternatively, a resistor element or a transmission line may be used instead of inductor element 1052.
In FET 1080, a source electrode is grounded, the input signal of the amplifier is input to a gate electrode, and a DC voltage is applied at a drain electrode through inductor element 1052, a resistor element, or a transmission line. The drain terminal is also an output terminal of the amplifier.
FIG. 3 is a waveform chart showing the relationship between input signal of the amplifier 1204 that is amplified by the class AB amplifier shown in FIG. 1 and DC voltage 1205 that is a supply voltage of the class AB amplifier shown in FIG. 1.
As shown in FIG. 3, in the class AB amplifier shown in FIG. 1, regardless of the amplitude of input signal of the amplifier 1204, DC voltage 1205 applied to power amplifier 1003 was constant. Thus, the class AB amplifier shown in FIG. 1 had a problem as regards a low power efficiency due to wasteful power caused by heat generation and so forth.
FIG. 4 is a block diagram showing an envelope tracking amplifier (ET amplifier) that solves such a problem.
In FIG. 4, the ET amplifier obtains an envelope signal (AM (amplitude modulation) signal) having the waveform of the input signal of the amplifier from an envelope detector (detector) or a base band itself. The AM signal is amplified by envelope amplifier 1009 and the amplified signal is input to power supply terminal 1003a. In addition, a signal containing a phase component (PM (phase modulation)) of the input signal of the amplifier is input to signal input terminal 1003b. Power amplifier 1003 amplifies the signal that is input to signal input terminal 1003b. 
FIG. 5 is a waveform chart showing the relationship between input signal of the amplifier 1207 amplified by the ET amplifier and supply voltage 1206. As shown in FIG. 5, since input signal of the amplifier 1207 is synchronized with supply voltage 1206, the lower the power of the input signal of the amplifier, the lower the supply voltage and the higher the power of the input signal of the amplifier, the higher the supply voltage. As a result, since the wasteful power decreases, the efficiency improves.
In the ET amplifier, a signal that is input to power amplifier 1003 (input signal of the amplifier) may be only a phase component signal (PM signal). Alternatively, that signal that is input to power amplifier 1003 may be a signal that contains the PM signal and the envelope component (AM signal). The former amplifier may be generally referred to as the polar modulation amplifier or envelope elimination and restoration (EER) amplifier, whereas the later amplifier may be generally referred to as the envelope tracking amplifier. However, since the amount of the AM signal in the signal that is input to power amplifier 1003 is a trivial matter, these amplifiers are collectively referred to as the envelope tracking amplifier (ET amplifier).
The efficiency of the ET amplifier is represented by the product of the efficiency of power amplifier 1003 and the efficiency of envelope amplifier 1009. Thus, not only the efficiency of the amplifier, but also the efficiency of the envelope amplifier needs to be improved. As a result, a circuit that has both a high efficient digital amplifier and a low pass filter (that operates as an integrating circuit) has been widely used as a constituent element of the envelope amplifier.
FIG. 6 is a schematic diagram showing an ET amplifier that includes an envelope amplifier provided with a digital amplifier and a low pass filter; and a power amplifier. In FIG. 6, the ET amplifier includes linear amplifier 1001 that is an analog amplifier, resistor element 1040, digital amplifier 1002, low pass filter 1007, and power amplifier 1003. Linear amplifier 1001, resistor element 1040, digital amplifier 1002, and low pass filter 1007 are included in envelope amplifier 1009. Digital amplifier 1002 and low pass filter 1007 are included in digital amplifier 1011.
FIG. 7 is a schematic diagram showing the ET amplifier shown in FIG. 6 in detail, more specifically, shows digital amplifier 1002 in detail.
FIG. 8 is a schematic diagram showing the ET amplifier shown in FIG. 6 in detail, more specifically, shows digital amplifier 1002 and low pass filter 1007 in detail.
In FIG. 7 and FIG. 8, digital amplifier 1002 is divided into comparator 1020, gate driver circuit 1021, and switching amplifier 1022. In FIG. 8, inductor element 1050 is used as low pass filter 1007.
Next, the operations of the ET amplifiers shown in FIG. 6, FIG. 7, and FIG. 8 will be described.
In these ET amplifiers, an AM signal is input to linear amplifier 1001. Comparator 1020 compares an output signal of linear amplifier 1001 with an output signal of digital amplifier 1011 (specifically, an output signal of low pass filter 1007). When the potential of the output signal of digital amplifier 1011 is higher than the potential of the output signal of linear amplifier 1001, comparator 1020 outputs a signal that represents “low”; when the potential of the output signal of linear amplifier 1001 is higher than the potential of the output signal of digital amplifier 1011, comparator 1020 outputs a signal that represents “high”.
The output signal of comparator 1020 is input to gate driver circuit 1021 that drives switching amplifier 1022. Although the output currents of gate driver circuit 1021 and switching amplifier 1022 are large, since they are inverter circuits, the output voltage of switching amplifier 1022 is equal to the output voltage of comparator 1020.
The output signal of switching amplifier 1022 is smoothened by inductor element 1050 that serves as low pass filter 1007. Thus, the output signal of inductor element 1050 becomes a signal having the same amplitude as does the original waveform (AM signal).
As a result, digital amplifier 1011 supplies most of the power that linear amplifier 1001 itself cannot supply with high efficiency, whereas analog linear amplifier 1001 compensates a portion that is insufficient in the digital signal. Consequently, an envelope amplifier that has high efficiency (70 to 90%) and high linearity can be realizes.
Since the efficiency of the entire amplifier is decided by the product of the efficiency of the envelope amplifier and the efficiency of the power amplifier itself, the efficiency of the envelope amplifier needs to be as high as possible. In the envelope amplifier shown in FIG. 8, resistor element 1040 is located in an output path of linear amplifier 1001. Thus, the loss of the signal power caused by resistor element 1040 adversely affects the efficiency.
The problem, in which the loss is caused by resistor element 1040, can be theoretically solved by decreasing the resistance of resistor element 1040. However, since the resistance required for resistor element 1040 becomes 1Ω or below, occasionally, 0.1Ω or below, it was difficult to lower the resistance to 1Ω or below because of a problem, in which the ease of design deteriorates, that will be described later.
Techniques that solve the problem in which a loss caused by resistor element 1040 have been proposed in Non-Patent Literature 1 and Non-Patent Literature 2.
FIG. 9 is a schematic diagram describing the technique that is presented in Non-Patent Literature 1.
In the technique presented in Non-Patent Literature 1, the last stage class AB amplifier that composes linear amplifier 1001 is divided into two class AB amplifiers 1023 and 1024. The earlier stage amplifier that composes linear amplifier 1001 (OTA (Operational Transconductance Amplifier)) 1025 is shared by amplifiers 1023 and 1024.
The ratio of gate widths of class AB amplifiers 1023 and 1024 in N:1 (N>>1) and the ratio of output currents thereof are N:1.
A first output signal of linear amplifier 1001 (an output signal of class AB amplifier 1023) is not input to power supply terminal 1003a through a resistor element and is also used as a feedback to linear amplifier 1001.
Since current that flows in class AB amplifier 1023 is larger than the current that flows in class AB amplifier 1024, the operation of class AB amplifier 1023 that depends on a feedback decides the operation of class AB amplifier 1024 that uses the same gate terminal as does class AB amplifier 1023. Thus, class AB amplifier 1024 operates similarly as does class AB amplifier 1023.
A second output signal of linear amplifier 1001 (an output signal of class AB amplifier 1024 is input to comparator 1020 through current-to-voltage conversion circuit 1026. The input signal is compared with a reference potential that is applied at terminal 1027 and the compared result is output as an output signal of the digital amplifier to power supply terminal 1003a. 
FIG. 10 is a schematic diagram describing the technique presented in Non-Patent Literature 2.
In the technique presented in Non-Patent Literature 2, two linear amplifiers 1001 and 1010 are used. An output terminal of one linear amplifier 1001 is connected to power supply terminal 1003a. Comparator 1020 compares an output signal of linear amplifier 1001 with an output signal of linear amplifier 1010. An output terminal of comparator 1020 is connected to power supply terminal 1003a through gate driver 1021, switching amplifier 1022, and inductor element 1050 that serves as a low pass filter.
Since an output terminal of linear amplifier 1010 is connected only to an input terminal of comparator 1020 and a feedback terminal of linear amplifier 1010, the output terminal is connected in a high impedance state. On the other hand, an output terminal of linear amplifier 1001 is connected to an output terminal of a digital amplifying section. Thus, the output signal of linear amplifier 1001 is affected by an output signal of the digital amplifying section.
By comparing the output signal of linear amplifier 1010 with the output signal of linear amplifier 1001, the amplifier shown in FIG. 10 operates similarly to the circuit shown in FIG. 8.